linux-kernel - [v2] lib/string: add memrchr function

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Date:   Wed, 13 Feb 2019 02:54:43 +0800
From:   Xiang Xiao <xiaoxiang781216@...il.com>
To:     gregkh@...uxfoundation.org, alexander.shishkin@...ux.intel.com,
        andriy.shevchenko@...ux.intel.com, linux-kernel@...r.kernel.org
Cc:     Xiang Xiao <xiaoxiang@...omi.com>
Subject: [v2] lib/string: add memrchr function

Here is the detailed description for memrchr:

void *memrchr(const void *s, int c, size_t n);

The memrchr() function is like the memchr() function, except
that it searches backward from the end of the n bytes pointed
to by s instead of forward from the beginning.

The memrchr() functions return a pointer to the matching byte
or NULL if the character does not occur in the given memory
area.

Signed-off-by: Xiang Xiao <xiaoxiang@...omi.com>
---
 include/linux/string.h |  1 +
 lib/string.c           | 21 +++++++++++++++++++++
 2 files changed, 22 insertions(+)

diff --git a/include/linux/string.h b/include/linux/string.h
index 7927b87..f380f4b 100644
--- a/include/linux/string.h
+++ b/include/linux/string.h
@@ -167,6 +167,7 @@ static inline void memcpy_flushcache(void *dst, const void *src, size_t cnt)
 	memcpy(dst, src, cnt);
 }
 #endif
+void *memrchr(const void *s, int c, size_t n);
 void *memchr_inv(const void *s, int c, size_t n);
 char *strreplace(char *s, char old, char new);
 
diff --git a/lib/string.c b/lib/string.c
index 38e4ca0..92914f6 100644
--- a/lib/string.c
+++ b/lib/string.c
@@ -964,6 +964,27 @@ void *memchr(const void *s, int c, size_t n)
 EXPORT_SYMBOL(memchr);
 #endif
 
+/**
+ * memrchr - Find a character in an area of memory.
+ * @s: The memory area
+ * @c: The byte to search for
+ * @n: The size of the area.
+ *
+ * returns the address of the last occurrence of @c, or %NULL
+ * if @c is not found
+ */
+void *memrchr(const void *s, int c, size_t n)
+{
+	const unsigned char *p = s + n;
+
+	while (n-- != 0) {
+		if ((unsigned char)c == *--p)
+			return (void *)p;
+	}
+	return NULL;
+}
+EXPORT_SYMBOL(memrchr);
+
 static void *check_bytes8(const u8 *start, u8 value, unsigned int bytes)
 {
 	while (bytes) {
-- 
2.7.4